The present invention relates to a process for improving the quality of propylene oxide.
Propylene oxide is widely used as precursor for preparing polyether polyols, which upon reaction with polyisocyanate compounds yield polyurethanes. Typically, methods for preparing polyether polyols involve reacting a starting compound having a plurality of active hydrogen atoms with propylene oxide, optionally together with one or more other alkylene oxides like ethylene oxide or butylene oxide. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 6 hydroxyl groups. Examples of such alcohols are glycols, glycerol, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, mannitol, etc. Usually a strong base like potassium hydroxide is used as a catalyst in this type of reaction.
The quality of the propylene oxide used to prepare the polyether polyol has significant impact on the quality of the polyurethane foams eventually obtained. Particularly the presence of poly(propylene oxide) is known to cause undesired effects in the polyurethane foam formation. Examples of such undesired effects are the occurrence of blow holes, low foam rise and even collapse of the foam formed. Particularly, in moulding applications the presence of poly(propylene oxide) in the propylene oxide used for preparing the starting polyether polyol may cause problems in terms of quality of the polyurethane foam.
The term xe2x80x9cpoly(propylene oxide)xe2x80x9d as used throughout the present specification refers to poly(propylene oxide) having a molecular weight of 2000 Dalton or higher as determined by polypropylene glycol-calibrated gel permeation chromatography.
Methods for manufacturing propylene oxide are well known in the art. Commercial production normally takes place via the chlorohydrin process or via the hydroperoxide process. In the latter process propene is reacted with an organic hydroperoxide. This hydroperoxide is either tert-butyl hydroperoxide or ethylbenzene hydroperoxide. In the first case tert-butyl alcohol is formed as a co-product (to be further converted into methyl tert-butyl ether), in the second case styrene is formed as the co-product. In the chlorohydrin process chlorine, propene and water are reacted to form propylene chlorohydrin, which is subsequently dehydrochlorinated with calcium hydroxide to form propylene oxide. For the purpose of the present invention it is immaterial which preparation route is used. Namely, in all processes poly(propylene oxide) is formed in undesirably high quantities. Moreover, it is known (e.g. from U.S. Pat. No. 4,692,535) that high molecular weight poly(propylene oxide) may be formed during storage or transport, for example upon contact with a metal, such as carbon steel.
Methods for improving the quality of propylene oxide via adsorption of poly(propylene oxide) are known in the art. Several adsorbents have been reported to be useful for this purpose. For instance, U.S. Pat. No. 4,692,535 discloses the use of activated carbon, charcoal or attapulgite as suitable adsorbents. In EP-A-0,601,273 non-calcined diatomaceous earth is mentioned as adsorbent for removing poly(propylene oxide). In JP-A-08/283253 zeolites and magnesia are mentioned as adsorbents. Suitable zeolites have a pore diameter between 3 and 10 xc3x85ngstrom, while the magnesia should suitably consist for at least 90 wt % of magnesium oxide.
Although the known adsorbents, and in particular activated carbon, perform satisfactorily in removing poly(propylene oxide) from propylene oxide, there is still room for improvement. The present invention aims to provide a process for improving the quality of propylene oxide by adsorption, wherein the adsorbent used has at least a similar performance in terms of poly(propylene oxide) removal as activated carbon.
According to U.S. Pat. No. 5,493,035 there are various difficulties associated with using activated carbon as the adsorbent for purifying propylene oxide, particularly during the initial or start-up phase of the activated carbon treatment. The adsorption of propylene oxide onto the activated carbon, namely, is highly exothermic and hence causes excessive temperature increases during said start-up. This has many undesired consequences, one of which is propylene oxide vaporisation and migration in the bed which in return causes secondary exotherms with very high temperatures. This is extremely hazardous and may even cause reactor damage according to U.S. Pat. No. 5,493,035. The solution proposed in U.S. Pat. No. 5,493,035 is a pretreatment of the activated carbon involving contacting this activated carbon with a glycol, such as propylene glycol.
It was envisaged that the adsorbent to be used in the process according to the present invention should not have the above risks associated with the use of activated carbon. On the other hand, the purification performance of the adsorbent to be used should be at least similar to that of activated carbon.
Accordingly, the present invention relates to a process for improving the quality of an propylene oxide contaminated with poly(propylene oxide), which process comprises the steps of:
(a) contacting the liquid propylene oxide with an adsorbent consisting of magnesium silicate and/or calcium silicate under such conditions that the amount of poly(propylene oxide) is reduced to the desired level, and
(b) recovering the purified propylene oxide product.
As has already been indicated above, the way in which the propylene oxide is prepared is immaterial to the present invention. Any known preparation process may be applied. The propylene oxide to be treated in the process according to the present invention may be the product directly obtained from the known preparation processes. Alternatively, said directly obtained propylene oxide also may have been subjected to conventional purification and recovery techniques before it is treated in accordance with the present invention. Assuming that the propylene oxide is produced in a hydroperoxide process, such purification and recovery techniques typically involve the removal of unreacted propene and organic hydroperoxide, by-products (like propane, aldehydes and alcohol) and other treating agents. In general, the propylene oxide stream to be treated in the process of the present invention consists for at least 95 wt % of propylene oxide.
The adsorbent is magnesium silicate, calcium silicate or a mixture of both. In principle the known, commercially available magnesium silicates and calcium silicates may be used. Preferred magnesium silicates are the synthetic ones, e.g. prepared by reacting a magnesium salt like magnesium sulphate with sodium silicate. Similarly, synthetic calcium silicates may be used. Typically, the magnesium and calcium silicates are used in their hydrated form, although the dehydrated or water-free silicates may also be used. The use of magnesium silicate as the adsorbent is preferred.
The adsorbent may be used as a powder to form a slurry with the propylene oxide or may be used in extruded form in a bed through which the propylene oxide is passed.
Accordingly, step (a) may in a first embodiment comprise contacting the liquid propylene oxide with a fine powder of the adsorbent. The average particle size of such powder will typically be in the range from 1 to 100 xcexcm, preferably from 2 to 40 xcexcm. Suitably, the adsorbent is dispersed in the liquid propylene oxide yielding a slurry. In this embodiment of the present invention, step (b) advantageously is a filtration step yielding a permeate (or filtrate) containing the purified propylene oxide product. The retentate, consequently, contains the adsorbent with poly(propylene oxide) adsorbed thereon. Filtration may be carried out by microfiltration methods known in the art. The filter used should have such openings that the adsorbent with poly(propylene oxide) adsorbed thereon cannot pass these openings. The exact filter to be used, accordingly, depends on the size of the adsorbent powder particles used. Suitable filters for instance include glass filters, plate filters and multi-tube filters like the Fundabac filters or the Contibac filters (Fundabac and Contibac are trade marks). The multi-tube filters generally comprise a vessel filled with vertically arranged filter elements distributed over a number of compartments, whereby each filter element is a tube of a porous material surrounded by a filter cloth. The slurry is passed through the vessel and the liquid purified propylene oxide is pressed through the filter cloth and the wall of the porous tube into said tube and is recovered at the end of said tube.
The amount of adsorbent used, when the adsorbent is used in powdered form, typically ranges from 0.01 to 20 wt % based on the amount of liquid propylene oxide treated. Preferably, the amount of adsorbent used is in the range of from 0.05 to 15 wt % based on liquid propylene oxide. In general, when using the adsorbent in powdered form, it is preferred to use as little adsorbent as needed to effectively remove poly(propylene oxide), and accordingly it is preferred to use at most 10 wt % and more preferably at most 5 wt % of adsorbent powder. The powder adsorbent may have a surface area of from 10 to 1000 m2/g, but preferably the surface area is at least 50 m2/g, more preferably at least 200 m2/g and even more preferably at least 400 m2/g.
In an alternative embodiment of the process according to the present invention step (a) comprises passing the contaminated propylene oxide over a bed of shaped particles of the adsorbent. These particles may have any shape conventionally used, including spheres, cylinders, stars, trilobes, quadrulobes, hollow cylinders or monoliths. Their size (diameter) typically is in the order of millimeters, such as from 0.1 to 5 mm. Cylinders typically have a length/diameter ratio of from 2 to 6, preferably 3 to 5. The porosity and surface area of such shaped particles should be such that the poly(propylene oxide) can be adequately adsorbed. A preferred porosity in terms of pore volume is from 0.1 to 3 ml/g, more preferably 0.2 to 2 ml/g and even more preferably 0.5 to 1.2 ml/g as determined by nitrogen adsorption. The surface area may suitably range from 150 to 800 m2/g, more suitably 200 to 600 m2/g and even more suitably 250 to 500 m2/g as determined by the BET method (ISO 9277: 1995(E)).
In case the particles of the adsorbent material, i.e. magnesium silicate or calcium silicate, are shaped using extrusion, the extrudates will typically comprise a binder material and the adsorbent material. Suitable binder materials include inorganic oxides like silica, magnesia, titania, alumina, zirconia and silica-alumina, of which silica is preferred. The weight ratio of binder to adsorbent material may vary from 10:90 to 90:10, suitably from 20:80 to 50:50.
The extrudates can be made by conventional extrusion techniques known in the art. Typically an extrusion mixture is prepared from powders of the solids (adsorbent and binder) and water by mixing and kneading the ingredients and passing this mixture into the extruder. Such mixture typically has a paste-like appearance. It is within the normal skills of those skilled in the art to optimise the mixing/kneading procedure to obtain an extrudable paste and to select the most appropriate extrusion conditions. Beside the adsorbent material, binder and water the extrusion paste will normally also comprise extrusion aids to improve the flow properties. Such extrusion aids are known in the art and include, for instance, aliphatic mono-carboxylic acids, polyvinyl pyridine, and sulfoxonium, sulfonium, phosphonium and iodonium compounds, alkylated aromatic compounds, acyclic monocarboxylic acids, fatty acids, sulfonated aromatic compounds, alcohol sulfates, ether alcohol sulfates, sulfated fats and oils, phosphonic acid salts, polyoxyethylene alkylphenols, polyoxyethylene alcohols, polyoxyethylene alkylamines, polyoxyethylene alkylamides, polyacrylamides, polyacryl amines, polyols, polyvinyl alcohols, acetylenic glycols and graphite. Burnout materials may also be used to increase the porosity of the final extrudate. Examples of burnout materials are polyethylene oxide, methylcellulose, ethylcellulose, latex, starch, nut shells or flour, polyethylene or any of the polymeric microspheres or microwaxes.
After extrusion the extrudates are dried and calcined. Drying may be effected at an elevated temperature, preferably up to 300xc2x0 C., more preferably up to 200xc2x0 C. The period for drying may vary, but will usually up to 5 hours, more suitably from 30 minutes to 3 hours. The drying may also be integrated with the subsequent calcination. Calcination is typically effected at an elevated temperature, preferably up to 1000xc2x0 C., more preferably from 200 to 800xc2x0 C., most preferably from 300 to 700xc2x0 C. Calcination of the extrudates is typically effected for a period of up to 5 hours, preferably from 30 minutes to 4 hours.
Following calcination the extrudates may be subjected to a treatment to neutralise any catalytically active acid sites still present after calcination or possibly formed on the surface of the extrudates during calcination. These acid sites, namely, could potentially promote the formation of poly(propylene oxide). Such treatment could, for example, involve immersing the calcined extrudates in water or subjecting them to a steaming treatment. For the purpose of the present invention a steaming treatment is preferred. Such steaming treatment can be carried out by the conventional methods, for instance, by contacting the calcined extrudates with low pressure steam of 120-180xc2x0 C. for 30 minutes up to 48 hours, suitably from 2 to 24 hours. If a water immersion or steaming treatment is carried out a drying step under mild conditions (i.e. at 30-100xc2x0 C.) is carried out.
The extrudates are suitably packed into a fixed bed and the liquid propylene oxide is then passed through this bed. This operation may be repeated several times by recycling the propylene oxide over the adsorbent bed or by passing the propylene oxide through a cascade of two or more fixed bed adsorption columns arranged in series. The purified propylene oxide product is recovered as the bottom stream leaving the adsorption bed or leaving the adsorption bed for the last time (when recycling) or leaving the final bed (when using a cascade of adsorption beds).
One suitable mode of operation is to use two adsorption columns with one column being used as a swing column. In this mode of operation one adsorption column is in operation while the other is bypassed, e.g. for replacement of the adsorbent material. Once the adsorption performance of the adsorbent in the column in operation reaches an undesirable low level, the other adsorption column with fresh adsorbent is taken into operation while the column with the (partly) xe2x80x9cdeactivatedxe2x80x9d adsorbent is taken out of operation for replacement of the adsorbent. In this way the adsorption treatment can be very effectively operated. Alternatively, a single adsorption column is used and is temporarily bypassed when the bed needs to be replaced. Given the huge volume of propylene oxide passed over the bed, the poly(propylene oxide) content of the propylene oxide not passed over the adsorption column will be greatly diluted by the large volume of treated propylene oxide. From a process economic perspective this latter option is preferred as it requires only one adsorption column.
The magnesium silicate or calcium silicate adsorbent may be pretreated with an organic liquid to minimise the adsorption heat which is generated when poly(propylene oxide) is adsorbed onto the adsorbent. Although the adsorption heat is already much less than when using activated carbon, a further decrease could be beneficial as it could save on cooling capacity when operating on a commercial scale. In general, cooling equipment is very expensive so if it is possible to dispense with such expensive equipment that would be advantageous. Suitable organic liquids which could be used for this purpose include a glycol, such as propylene glycol, as disclosed in U.S. Pat. No. 5,493,035 (discussed hereinbefore), but preferably an organic liquid selected from ethylbenzene, 1-phenylethanol (methylphenyl carbinol), methylphenyl ketone or a mixture of two or more of these is used. The pretreatment typically involves contacting the adsorbent with the organic liquid for sufficient time to adsorb sufficient organic liquid onto the adsorbent.
The conditions applied in step (a) should preferably be such that the concentration of poly(propylene oxide) is reduced to 0.5 mg/l or less, more preferably to 0.2 mg/l or less. Furthermore, the conditions should be such that the propylene oxide remains in the liquid state. Thus, at atmospheric pressure temperatures from 0xc2x0 C. up to 34xc2x0 C. may be applied. Suitably, step (a) is carried out at a temperature in the range of from 5 to 30xc2x0 C. The pressure is not particularly critical and will normally be in the range of from 0.5 to 10 bar, more suitably from 0.5 to 4 bar. Operating at atmospheric conditions is usually most preferred. The contact time between adsorbent and propylene oxide should be sufficient to achieve the target level of poly(propylene oxide) in the final propylene oxide product. Typically, contact times may vary from 1 minute to several hours, but for practical reasons contact times of 5 minutes to 2 hours are preferred. In a fixed bed operation the liquid hourly space velocity will suitable be from 0.5 to 10 hrxe2x88x921.